[keras] removes old function definition

keras.py

@@ -80,13 +80,6 @@ class KFeed(object):
     def balance(self, src):
         return src
 
-    def balance_weights(self, weights):
-        if isinstance(weights, SKDict):
-            sums = weights.map(np.sum)
-            ref = np.mean(list(sums.values()))
-            weights = weights.__class__({k: weights[k] * (ref / s) for k, s in sums.items()})
-        return weights
-
     def kfeed(self, src, **kwargs):
         src = src.only(*self.all)
         bal = self.balance(src)